Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as an energy source. Among these secondary batteries, a great deal of research and study has been focused on a lithium secondary battery having high-energy density and high-discharge voltage and thus some of such lithium secondary batteries are commercially available and widely used.
However, conventional lithium secondary batteries are susceptible to the high-risk of ignition/explosion arising from heating of batteries by IR-induced heat generation when large amounts of electrical current flow within a short period of time due to various factors such as exposure to high temperatures, overcharge, external short-circuiting, nail penetration, localized crushing and the like. Elevation of the battery temperature results in facilitation of reaction between the electrolyte and electrodes. As a consequence, occurrence of reaction heat is additionally accompanied by elevation of the battery temperature which in turn further accelerates the reaction between the electrolyte and electrodes. Therefore, the temperature of the battery rises sharply, thereby further accelerating the reaction between the electrolyte and electrodes. Due to being caught in such self perpetuating cycle, thermal runaway, which causes sharp elevation in the battery temperature, occurs, and ignition of the battery may take place if the battery temperature rises over a specified range. In addition, the reaction between the electrolyte and electrodes leads to generation of gases which in turn results in increased internal pressure of the battery and consequently the battery undergoes explosion at pressure exceeding a predetermined range. As such, it can be said that the risk of ignition/explosion is the most fatal disadvantage of lithium secondary batteries.
Therefore, an essential requirement, which should be considered for the development of lithium secondary batteries, is to secure the battery safety. As attempts to secure the battery safety, there may be mentioned a method of disposing some elements externally of the cell and a method of using materials inside the cell. The former method involves elements such as a Positive Temperature Coefficient (PTC) device using temperature changes, a Circuit Interrupt Device (CID), a protection circuit for controlling voltage and current, and Safety Vent using changes in internal pressure of the battery, whereas the latter method involves addition of materials which can undergo physical, chemical or electrochemical changes in response to changes of internal temperature, voltage or current of the battery.
Devices disposed externally of the cell exert their functions by using changes of temperature, voltage, current and internal pressure of the battery and therefore offer accurate shutdown, but suffer from disadvantages such as a need for additional installation processes and space and limited application of CID devices to cylindrical batteries only. Further, it is also known that such external devices do not sufficiently exert their protection function when a rapid response time is required as in external short-circuiting, nail penetration, localized crushing and the like.
Meanwhile, one of the methods of using material incorporated internally of the cell is to add additives capable of improving safety to electrolytes or electrodes. For example, there are known various methods as follows: for example, a method of adding materials, which undergo electrochemical polymerization under overcharge conditions or the like, to electrolytes, whereby, upon overcharge, the resulting polymerization products form an immobilized film on the electrodes or the electrolytes are solidified, thereby inhibiting abnormal operation of the battery, and a method of inhibiting abnormal operation of the battery by increasing resistance of the electrode via swelling of an additive volume upon exposure to high temperatures or overcharge. Such chemical safety measures have advantages such as no need for additional installation process and space and feasible application thereof to all kinds of batteries, but suffer from problems associated with incapability to provide reliable operation and deterioration of battery performance due to addition of materials.
As such, there is an urgent need for the development of a novel chemical safety measure to prevent the risk of ignition/explosion without significant deterioration of general performance of the battery upon overcharge thereof.